Message in a Bottle
If there's a quintessential romantic image that can encapsulate both mystery and intrigue, it's a bottle with a message inside. There's a lot of fun to be had with this image that we've all seen in movies, books, etc.
One way to make use of this idea is to prepare a bottle and send it out to sea (or down a river), hoping it reaches somewhere more or less distant, gets picked up, and someone sends us a reply (you'll logically need to include your details in the message :P). This isn't a conventional idea, but we're sure to have fun preparing the bottle and letter and fantasizing about who might find it. If we do this together, we'll have twice the fun. There's also the chance we might pleasantly surprise someone who happens to be swimming. Actions like these make the world a better place because of their unexpected nature.
Another great option is to cheat a little and prepare a bottle with a lovely letter inside for the person you want to surprise. The idea would be to go with this person to the beach or river and have them 'accidentally' find the bottle. Some skeptics might think it's impossible for such a surprise to work, that the most likely scenario is they'll suspect us as soon as they find the bottle. To those, I encourage you to trust me; I assure you the idea works. Anyway, if you want to be more certain, you can ask a friend to act as a decoy, making the scene as 'natural' as possible.
A third, much more homey idea is to use the bottle and let it float in the bathtub, waiting for a romantic bath with your partner. You're sure to get a big smile when they find it and read the message inside.
Regarding the execution of the idea itself, my recommendation is to use a wine bottle. To remove the label, soak it in hot water, and it will come off easily. If you want to stay true to the image we all have of a message in a bottle, get a cork stopper and, if necessary, adapt it to the bottle. Secure it well using some plastic (a piece of a bag will do) because it's essential that no water gets into the bottle. Otherwise, the idea will sink to the bottom along with it. It wouldn't be a bad idea to wrap the letter in something waterproof to protect it from possible leaks.
Another romantic touch can be to print the message in an old-style font and burn the edges a little. If you also want to yellow the paper to make it look like a scroll, pour some cola on it without soaking it, just to moisten it, and pass a candle underneath so the liquid dries without burning the paper. Do a few experiments beforehand because it's hard to get it right on the first try.
Finally, I encourage you to combine this idea with another... perhaps a bottle containing a map leading to treasure? Let your imagination run wild.